This invention relates to skid-steering vehicles such as tractors and military tanks where the vehicle is driven by two ground engaging members which may be two tracks or two sets of wheels, placed on each side of the vehicle, and steered by an imposed difference in speeds of the tracks or sets of wheels.
The present invention provides a transmission consisting of prime movers, hydraulic pumps, hydraulic motors, associated circuitry and a gear box that provides new capabilities in such skid-steering vehicles both in improved high speed performance and in improved vehicle lay-out and control characteristics while still meeting necessary space and weight criteria.
Many low speed tractors and the like have separate hydraulic motors driving each track through gearing to provide both steering and traction. However, they are inherently limited in speed, for any reasonable input power, because steering requires that the tracks or sets of wheels be skidded at any speed, and the power needed to skid the tracks or sets of wheels is excessive at high vehicle speeds.
High speed skid-steer vehicles, such as those used for military applications, overcome this problem by using differential epicyclic gearing `superposed` on the drive train to each track or set of wheels with one element of each such differential being independently driven by, commonly, a hydraulic motor to provide steering. Usually the input drive is connected to the annulus gear of both differential epicyclic gear sets, while the output to the tracks or sets of wheels is connected to the planet frame, the steering input being connected to the sun gear.
During turning at speed, the required torque difference is generated to skid the tracks or sets of wheels, with the turning power, often many times the drive power, being almost entirely transmitted directly from the inside track or set of wheels to the outside track or set of wheels through the superposed gears, the drive transmission continuing only to transmit drive power from the prime mover.
Typically, such a transmission would consist of a mechanical, hydrokinetic-mechanical or hydrostatic-mechanical transmission connecting the prime mover to both annuli for traction purposes. The prime mover also drives a hydraulic pump which in turn drives a hydraulic motor driving the two sun gears, through gearing in opposite directions. If the motor is held stationary, the track or wheel set speeds will be equal and the vehicle will tend to travel in a straight line. If the motor is rotated, a speed difference will be superimposed on the tracks or sets of wheels causing the vehicle to turn.
Another variety of turn control described in the patent literature, but to the inventor's knowledge not used in practice, attaches a hydraulic pump/motor to one element of each of the superposed epicyclic gears, without an engine driven pump, thus providing a torque and speed ratio between these elements. Such an arrangement would not provide the same ease of control as the more common system described in the previous paragraph.
These basic drive and steering systems, with many detailed variations are described in many patents, the most relevant to the present invention are:
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